This application relates to the co-production of aromatics, especially C.sub.6 -C.sub.8 aromatics, and olefins, especially C.sub.2 -C.sub.4 olefins, from paraffinic feedstocks (e.g., Udex raffinate) by converting said feedstocks over a catalyst of somewhat low activity, said catalyst comprising ZSM-5 or ZSM-11.
The Cattanach U.S. Pat. No. 3,756,942, the entire disclosure of which is expressly incorporated herein by reference, discloses a process for converting paraffinic feedstocks over zeolites such as ZSM-5 to produce a variety of hydrocarbon products. The underlying chemistry involved in this conversion is extremely complex. More particularly, a number of simultaneous and sometimes competing reactions take place to produce a variety of products which can, in turn, be reacted to form still different products. These possible reactions include cracking of paraffins, aromatization of olefins, and alkylation and dealkylation of aromatics. Products from the conversion of C.sub.5.sup.+ paraffinic feedstocks over ZSM-5 include C.sub.6 -C.sub.8 aromatics, C.sub.2 -C.sub.4 olefins, C.sub.9.sup.+ aromatics and C.sub.1 -C.sub.8 paraffins. Of these products the C.sub.6 -C.sub.8 aromatics and C.sub.2 -C.sub.4 olefins are most desired.
C.sub.6 -C.sub.8 aromatics, e.g. benzene, toluene, xylene and ethylbenzene, also known collectively as BTX, are valuable organic chemicals which can be used in a variety of ways. Since BTX has a high octane value it can be used as a blending stock for making high octane gasoline. By way of contrast, C.sub.9.sup.+ aromatics (i.e. aromatic compounds having at least 9 carbon atoms) tend to have a relatively low octane value.
C.sub.2 -C.sub.4 olefins, e.g., ethylene, propylene and butene, are also valuable organic chemicals which can be used to form polymers. By way of contrast, C.sub.1 -C.sub.3 paraffins (i.e. methane, ethane and propane), particularly in admixture, are less valuable chemicals which are generally used for fuel.
The acid catalytic activity of aluminosilicate ZSM-5 is proportional to aluminum content in the framework of the zeolite. The more aluminum in the ZSM-5 framework, the greater the acid catalytic activity of the ZSM-5, particularly as measured by alpha value. Note the article by Haag et al, "The Active Site of Acidic Aluminosilicate Catalysts," Nature, Vol. 309, 14 June 1985, pp. 589-591, especially FIG. 2 on page 590 thereof. ZSM-5 with very little framework aluminum and correspondingly low acid catalytic activity can be prepared from reaction mixtures containing sources of silica and alumina, as well as various organic directing agents. For example, the Dwyer et al U.S. Pat. No. 3,941,871, the entire disclosure of which is expressly incorporated herein by reference, describes the preparation of ZSM-5 from a reaction mixture comprising silica, tetrapropylammonium ions and no intentionally added alumina. The alumina to silica molar ratio of the ZSM-5 produced by this method may be less than 0.005.
U.S. Pat. No. 4,341,748, the entire disclosure of which is expressly incorporated herein by reference, describes the preparation of ZSM-5 from reaction mixtures which are free of organic directing agents. However, the reaction mixture for making this organic-free form of ZSM-5 is restricted to silica to alumina molar ratios of 100 or less. Consequently, this organic-free synthesis tends to produce ZSM-5 having a relatively high acid catalytic activity (e.g., alpha value) in comparison with zeolites prepared by the method of the Dwyer et al U.S. Pat. No. 3,941,871.
In accordance with an aspect of the present invention, it has been discovered that the selectivity to more valuable C.sub.6 -C.sub.8 aromatics and C.sub.2 -C.sub.4 olefins can be increased by converting C.sub.5.sup.+ paraffins over catalysts comprising ZSM-5, said catalysts having a relatively low alpha value.